The present invention relates to a method of redrawing a predrawn coated metal can with organic film.
Conventional methods of can forming are to draw and redraw (drawn-and-redrawn "DRD" can) or draw and iron (drawn-and-ironed "DI" can) two piece cans in which the sides and base are integrated. In addition to these, a drawn-thin/redrawn can ("DTR" can) is known. Since the DRD can is formed by drawing and redrawing, the wall thickness of the can is thick in proportion to the height of the can. For this reason, the DRD technique is generally used to make low height cans in consideration of cost effectiveness. The thickness of a DI can formed by ironing subsequent to the process of drawing is usually about one third of the starting wall thickness and therefore, these cans can economically be used in applications where the can is of relatively high height.
As a comparison between the DRD and DI cans, the former is drawn from a metal sheet initially coated with an organic film, while the latter is coated with an organic film after the ironing process. This results from the fact that the degree of processing and the stress intensities generated by the two different methods of processing are substantially different. If an organic film is applied to the metal sheet before processing of a DI can, which is subject to a much higher reduction ratio and thus the application of a much higher applied surface pressure, there may be damage of inner and outer organic films and jamming of the die with the organic material of the films. This tends to make this method of processing unsatisfactory.
The DTR can is formed by using a redrawing die with a smaller shoulder radius. Bending and bending back of the can wall are performed at this shoulder by applying a high tension to thin the wall thickness of the can. In the DTR method, the can wall is stretched by a process very similar to drawing, and again the wall thickness is made a little thinner than the starting thickness because the can wall is stretched during the process. Moreover, as the surface pressure applied on the can wall between the die and the punch is not so high, the load on the organic film is also not so high and therefore, damage of the organic film is unlikely. This makes it preferable to apply the organic film to the metal sheet prior to processing. However, the processing for the DTR can is based substantially on a tension force, which has a tendency to cause defects in or fractures of the wall, and so there is a disadvantage that the reduction ratio which can be reliably achieved is much smaller than in the case of a DI can.
As mentioned above, the DRD, DI and DTR cans have respective characteristics, although they each have particular problems. One of the objects of at least the preferred embodiments of the present invention is to provide a method of reducing the wall thickness of a predrawn can made from a metal sheet having a coating of organic film, with a high reduction ratio, by completing the processes of redrawing, stretching and ironing under certain conditions. There is thus disclosed herein a technique of can processing for forming a can which will have characteristics of both the DI and the DTR cans.
A known DTR can processing technique is disclosed in GB-A-2216052. Another known technique incorporating stretching and a small amount of ironing carried out at the same time as redrawing, is disclosed in GB-A-2061790. The technique of GB-A-2061790 requires the ironing process to be lightly performed with an aim of just obtaining a uniform wall thickness, where the reduction ratio depends upon a ratio of wall thickness to radius of die shoulder, that is, the required thinning of the can is executed by the DTR process. For this reason, the technique of GB-A-2061790 does not provide a high reduction ratio. It is directed towards the thickness of the can wall being made uniform throughout its height, and the end portion of the can wall remains to be flanged in the redrawing process, without being drawn.
Ideally, to reduce costs, the shell of a can should be thinned as much as possible and the top end portion of the shell should be thicker for subsequent neck-in processing (reduction of the diameter of the can at the end portion). The technique of GB-A-2061790 does not achieve this. According to the disclosure, if the can wall is thinned for weight reduction purposes, then it will be difficult to accomplish the subsequent neck-in processing successfully since the can wall is made uniform in thickness throughout its height. If, on the other hand, the can wall is made thicker in consideration of neck-in processing, then the benefits of weight reduction will be lost. Hence, the relationship between formability and weight reduction have to be offset against each other.
Thus, to summarise the prior art, the DI can processing is the most typical method of manufacturing a two-piece can having a relatively high can height, and is capable of thinning the can wall with a high ratio. However, it is difficult to apply an organic film coating to the metal sheet prior to processing because of possible damage to the film. With regard to DTR can processing, it is possible to apply an organic film to the metal sheet prior to processing, but it is difficult to thin the can wall to a high ratio.